AsyncTask

Class Overview

AsyncTask enables proper and easy use of the UI thread. This class allows to
perform background operations and publish results on the UI thread without
having to manipulate threads and/or handlers.

AsyncTask is designed to be a helper class around Thread and Handler
and does not constitute a generic threading framework. AsyncTasks should ideally be
used for short operations (a few seconds at the most.) If you need to keep threads
running for long periods of time, it is highly recommended you use the various APIs
provided by the java.util.concurrent package such as Executor,
ThreadPoolExecutor and FutureTask.

An asynchronous task is defined by a computation that runs on a background thread and
whose result is published on the UI thread. An asynchronous task is defined by 3 generic
types, called Params, Progress and Result,
and 4 steps, called onPreExecute, doInBackground,
onProgressUpdate and onPostExecute.

AsyncTask's generic types

Progress, the type of the progress units published during
the background computation.

Result, the type of the result of the background
computation.

Not all types are always used by an asynchronous task. To mark a type as unused,
simply use the type Void:

private class MyTask extends AsyncTask<Void, Void, Void> { ... }

The 4 steps

When an asynchronous task is executed, the task goes through 4 steps:

onPreExecute(), invoked on the UI thread before the task
is executed. This step is normally used to setup the task, for instance by
showing a progress bar in the user interface.

doInBackground(Params...), invoked on the background thread
immediately after onPreExecute() finishes executing. This step is used
to perform background computation that can take a long time. The parameters
of the asynchronous task are passed to this step. The result of the computation must
be returned by this step and will be passed back to the last step. This step
can also use publishProgress(Progress...) to publish one or more units
of progress. These values are published on the UI thread, in the
onProgressUpdate(Progress...) step.

onProgressUpdate(Progress...), invoked on the UI thread after a
call to publishProgress(Progress...). The timing of the execution is
undefined. This method is used to display any form of progress in the user
interface while the background computation is still executing. For instance,
it can be used to animate a progress bar or show logs in a text field.

onPostExecute(Result), invoked on the UI thread after the background
computation finishes. The result of the background computation is passed to
this step as a parameter.

Order of execution

When first introduced, AsyncTasks were executed serially on a single background
thread. Starting with DONUT, this was changed
to a pool of threads allowing multiple tasks to operate in parallel. Starting with
HONEYCOMB, tasks are executed on a single
thread to avoid common application errors caused by parallel execution.

public
AsyncTask()

Public Methods

public
final
boolean
cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task. This attempt will
fail if the task has already completed, already been cancelled,
or could not be cancelled for some other reason. If successful,
and this task has not started when cancel is called,
this task should never run. If the task has already started,
then the mayInterruptIfRunning parameter determines
whether the thread executing this task should be interrupted in
an attempt to stop the task.

See Also

Executes the task with the specified parameters. The task returns
itself (this) so that the caller can keep a reference to it.

Note: this function schedules the task on a queue for a single background
thread or pool of threads depending on the platform version. When first
introduced, AsyncTasks were executed serially on a single background thread.
Starting with DONUT, this was changed
to a pool of threads allowing multiple tasks to operate in parallel. Starting
HONEYCOMB, tasks are back to being
executed on a single thread to avoid common application errors caused
by parallel execution. If you truly want parallel execution, you can use
the executeOnExecutor(Executor, Params...) version of this method
with THREAD_POOL_EXECUTOR; however, see commentary there for warnings
on its use.

See Also

Executes the task with the specified parameters. The task returns
itself (this) so that the caller can keep a reference to it.

This method is typically used with THREAD_POOL_EXECUTOR to
allow multiple tasks to run in parallel on a pool of threads managed by
AsyncTask, however you can also use your own Executor for custom
behavior.

Warning: Allowing multiple tasks to run in parallel from
a thread pool is generally not what one wants, because the order
of their operation is not defined. For example, if these tasks are used
to modify any state in common (such as writing a file due to a button click),
there are no guarantees on the order of the modifications.
Without careful work it is possible in rare cases for the newer version
of the data to be over-written by an older one, leading to obscure data
loss and stability issues. Such changes are best
executed in serial; to guarantee such work is serialized regardless of
platform version you can use this function with SERIAL_EXECUTOR.

This method must be invoked on the UI thread.

Parameters

exec

The executor to use. THREAD_POOL_EXECUTOR is available as a
convenient process-wide thread pool for tasks that are loosely coupled.

Returns

public
final
boolean
isCancelled()

Returns true if this task was cancelled before it completed
normally. If you are calling cancel(boolean) on the task,
the value returned by this method should be checked periodically from
doInBackground(Object[]) to end the task as soon as possible.

Returns

true if task was cancelled before it completed

See Also

Protected Methods

protected
abstract
Result
doInBackground(Params... params)

Override this method to perform a computation on a background thread. The
specified parameters are the parameters passed to execute(Params...)
by the caller of this task.
This method can call publishProgress(Progress...) to publish updates
on the UI thread.